Coolent overflow question

About two weeks ago I noticed my overflow bottle was empty...so I topped it off. Fast forward a couple weeks, I was doing some work under the hood tonight and noticed it was empty again. What gives...low coolent level? I opened the radiator cap and could see coolent. My cruiser has not been running hot at all...the needle sits about 1/4 the way up on the temp guage; although, I'm not real sure how accurate it is. It has been hot here the last few days...could that have something to do with it? Where is the coolent going?

You will slowly loose water since it will evaporate when it is hot so just adding plain water to the overflow bottle is good. The gylcol in the coolant will not evaporate. I am not sure how much is normal but if it is not leaking out then the water is evaporating or you are working out some air pockets in the cooling system.

I can't keep any fluid in my overflow bottle either. For a while I would check the radiator level (when cool) and it was full to the cap every time. I'd fill the bottle half full and it would be empty within a month. I've never found any trace of external leakage but I suspect the problem is with either the bottle itself or the hose connecting it to the rad. When the engine is hot there is a small amount of coolant in the bottle.

Speaking of overflow bottles... I just changed the coolant on my SAAB and was impressed with their arrangement. The overflow bottle is pressurized along with the rad - there isn't a rad cap between the rad and the overflow bottle. The bottle has a pressure cap on it but it doesn't have to vent and recover coolant the way most conventional systems do.

I have the same problem -
My mech says it might be the filler cap because if it doesn't hold the pressure correctly it can leak. He suggested I replace the cap - I will know in a few weeks if this is the fix because the truck is at shop right now.
-mg

I had the same problem in my pickup and noticed that the PO had swapped the hoses to the overflow tank cap. The hose from the rad was connected to the overflow intake without the internal hose. So pressure from the rad built up, entered the overflow tank, and forced the liquid out the tank overflow line since it's hose was immersed in the coolant.